I. Field of the Invention
The present invention relates generally to crankshafts and, more particularly, to a method for making a multi-piece crankshaft.
II. Description of the Prior Art
Crankshafts of the type used in reciprocal piston engines and compressors typically comprise a main shaft having at least two axially aligned shaft sections with a crankpin extending between the facing ends of the shaft sections. The crankpin is oriented so that its axis is parallel to, but radially spaced from, the axis of the main shaft. Counterweights are then provided at the junction of each end of the crankpin with the main shaft thus forming the crankshaft.
There have been many different ways for manufacturing the previously known crankshafts. In one conventional method, the entire crankshaft is formed as a one-piece casting. After forming the casting, the casting is machined, typically by grinding, in order to form the crankshaft.
The previously known one-piece crankshafts, however, are disadvantageous in several different respects. One disadvantage of the previously known one-piece crankshafts is that the casting process and subsequent machining of the casting is a relatively expensive process requiring specialized and expensive equipment.
A still further disadvantage of the previously known one-piece crankshafts is that, since the crankshaft is formed from a one-piece casting, the entire casting must be made from the same material. This disadvantageously increases the overall material cost for the previously known one-piece crankshafts since the crankpins and main shaft must be constructed from a high strength steel while, conversely, it is unnecessary to construct the counterweights from a high strength material.
A still further disadvantage of the previously known one-piece crankshafts is that, in order to connect the piston rod to the crankpin, the connecting rod must necessarily utilize a split ring connector, i.e. two semi-circular shells which are secured together, typically by bolts. Such split ring connectors for the piston rods are necessarily more expensive than a single piece connector for a piston rod.
There have, however, been previously known crankshafts that are constructed from separate pieces, i.e. separate main shaft segments, separate counterweights and separate crankpins. Various methods, such as press fit splines, shaft deformation and the like, have been utilized by these previously known multi-piece crankshafts to mechanically connect the components of the crankshaft together.
Even though these previously known multi-piece crankshafts are suitable for many reciprocating piston applications, many crankshaft applications require that the various components of the crankshaft be oriented within very small manufacturing tolerances relative to each other. These tolerances include, inter alia, the concentricity between the segments of the main shaft, the parallelism of the axis of the crankpin to the axis of the main shaft, and also the radial spacing between the axis of the crankpin and the axis of the main shaft. As a practical matter, it has not been possible to manufacture these multi-piece crankshafts with the small manufacturing tolerances required by certain applications on a consistent and cost effective basis. In particular, it is extremely difficult to maintain the concentricity of the segments of the main shaft during the attachment of the counterweights and crankpin. Likewise, it has proven impractical to maintain the parallelism between the axis of the crankpin and the main shaft where small manufacturing tolerances are required.